Recently, there has been interest in the possibility that "differentiation-inducers" may have utility in the treatment of some malignancies. This concept is predicated on the belief that some malignancies are a result of a block in differentiation which if relieved would result in a more differentiated and therefore more benign condition. As a concept for therapy this approach holds the further promise that induction of differentiation could not only relieve the tumor burden but also increase the number of functional cells, that at least for some malignancies, an absence of is a major complication. HL60 has been a useful model system in the search for substances that are active as inducers of differentiation. HL60 is induced to differentiate to granulocyte-like cells by incubation with retinoic acid (RA), DMF, and DMSO or into monocyte/macrophage-like cells by incubation with 1,25-dihydroxyvitamin D3 and TPA. Of the many compounds that induce differentiation of HL60, RA has probably the most promise of being of use in the clinic. To the extent that it is possible, results in vitro should suggest treatments in vivo. To this end we have studied the differentiation effect of RA in nude mice carrying a transplantable HL60 tumor. There was no increase in life-span of treated animals and no evidence that tumor cells were induced to differentiate. These results were even more surprising because the tumor cells, established in culture and designated HL60/MRI, were found to be 100-fold more sensitive to RA in vitro than the parent HL60. In addition, RA induces HL60/MRI to differentiate to monocytoid cells. RA binds to serum albumin in a specific manner with an equilibrium constant of association of 10-6M-1. Thus, even at pharmacological concent concentrations of 1 MuM all of the circulating RA is bound to albumin. This binding may be a major factor for the absence of antitumor activity of RA in vivo as serum and/or serum albumin inhibit RA induced differentiation of HL60 in vitro.